1. Field of the Invention
The present invention relates to a method and an apparatus for inspecting a phosphor screen of a panel, an electron gun or a shadow-mask making up members of a cathode-ray tube in the processes of manufacturing the cathode-ray tube.
2. Description of Related Art
A phosphor screen of a cathode-ray tube is generally fabricated by forming a thin film of phosphor material on an inner surface of a panel glass and forming a metal thin film called a metal-back thereon. The metal-back is intended to prevent a phosphor screen from being charged by an electron beam irradiated from the electron gun on the one hand and to improve the brightness by reflecting the light emitted from the phosphor material toward the front of the panel face on the other. The metal-back, which is typically formed to have the thickness of about several hundred nm in such a manner as to allow the transmission of a sufficient quantity of electron beam to excite the phosphor material, is made of aluminum in many cases.
Now, defects of the cathode-ray tube are often caused duping the manufacturing process of the phosphor screen. The dropout of the phosphor film or the metal-back, if any, constitutes a direct cause of the screen defect. The improper or uneven film thickness, if not the dropout, leads to brightness irregularities causing a deteriorated screen brightness. Also, an unsatisfactory preparation of the phosphor material of course results in a failure to secure an illumination of predetermined chromaticity. As described above, the process for fabricating the phosphor screen requires utmost attention, and the inspection of the quality of the phosphor screen thus fabricated is an indispensable process for quality control.
The inspection of the phosphor screen plays an important role in the manufacture of the color picture tube of all the cathode-ray tubes. In a color picture tube for the typical home-use television receiver, for example, three types of phosphor materials for emitting the three primary colors of green, blue and red are arranged in stripes on the phosphor screen. These three phosphor materials are impinged upon by three electron beams independently of each other thereby to form an image of an arbitrary chromaticity and brightness. A shadow-mask is arranged at the intersection of the tracks of the three electron beams. The shadow-mask has a multiplicity of apertures corresponding to respective phosphor pixels making up the phosphor screen, so that the expansion of the electron beams emitted from the electron gun and bombarding the phosphor screen is limited and the appropriate color is selected by the incident angle of the three electron beams entering the apertures. Further, in order to prevent the entrance of the electron beams into adjacent phosphor pixels and thus to improve the image contrast, a black matrix made of light-absorbing black-body film is inserted in the clearances between the phosphor pixels. The black matrix is formed through the processes of applying resist, exposure, development, applying graphite, etching and development in that order. The phosphor film, on the other hand, is formed by repeating the processes of applying slurry, exposure and development for each color.
As described above, the phosphor screen of the color picture tubes which has a complicated construction and complicated fabricating processes construction as compared with the monochromatic cathode-ray tube, requires a quality control procedure based on the proper inspection methods in view of the fact that defects peculiar to the color picture tube would present themselves such as the mixed colors on the screen which might be caused from a dimensional error of the phosphor screen pattern.
The conventional methods of inspecting the phosphor screen are roughly of two types. One is the inspection of the final products, in which after the cathode-ray tube is completed as a bulb through all the manufacturing processes, the electron beams drawn from the electron gun sealed in the bulb are irradiated for scan on the phosphor screen, while observing the illumination in the process.
The second method is such that after forming the phosphor screen on the inner surface of the panel glass, light is irradiated on the phosphor screen to observe the illumination or the light transmitted through the phosphor screen. The phosphor screen inspection method disclosed in Japanese Patent Application Laid-Open No. 58-35445 (1983), for example, causes the phosphor screen to illuminate by irradiating the ultraviolet ray from a source like the black lamp on the panel formed on the phosphor screen thereby to detect the presence or absence of a defect of the phosphor material. In the method of inspecting the phosphor screen disclosed in Japanese Patent Application Laid-Open No. 1-227331 (1989), on the other hand, the white light of a high-pressure mercury lamp is irradiated from the back of the panel formed on the phosphor screen. This method is intended for detecting a defect by observing the transmitted light taking advantage of the fact that the light transmittivity is higher for the portion lacking the phosphor material or the metal-back.
In addition to the phosphor screen inspection described above, the inspection of the shadow-mask and the electron gun is a process indispensable for assuring the quality control in the manufacture of the cathode-ray tube. The shadow-mask, after press-formed, is checked for the workmanship, irregularities and appearance of the curved surfaces and is mounted on the panel. The electron gun, on the other hand, are assembled after inspection for dimensional accuracy of each unit of the electrode parts, and through the inspection of assembly accuracy including the inter-electrode distance, is sealed in the tube. The characteristic inspection of the shadow-mask and the electron gun is carried out as an item of the final product inspection after the cathode-ray tube is completed as a bulb.
The aforementioned conventional methods of inspecting the component parts of the cathode-ray tube including the panel phosphor screen, the shadow-mask or the electron gun poses the problems described below.
First, in the method of inspection conducted by causing the phosphor screen to illuminate after completing the cathode-ray tube as a bulb, the problem is posed that the inspection is possible only after a number of processes including the attaching the panel and the funnel, sealing the electron gun, exhausting the tube interior, the vacuuming and aging of the cathode, following the forming of the phosphor screen on the panel. As a result, in the event of an unexpected defective process of forming the phosphor screen, a long time is taken before discovery of the defect by inspection. All the bulbs charged in the production line in the process are unavoidably discarded as defective products, thereby leading to a great loss. This is not limited to the phosphor screen alone, but a very serious problem is caused similarly in the case where a defect is discovered during the manufacture of the electron gun or the shadow-mask as well.
Also, even if a defect is discovered by the screen inspection on a completed bulb, the job of specifying the defective component member causing the particular defect is a long, tedious one, thereby making it difficult to take a remedial action promptly.
In the method of observing the illumination or the transmission of the light irradiated on the panel forming the phosphor screen, on the other hand, the inspection can be carried out immediately after the process of forming the phosphor screen. This method is therefore superior to the above-mentioned method in that a defect is detectable at an earlier time, but has the disadvantage that the inspection items are considerably limited. In other words, according to the method of inspection by light irradiation, in which the electron beam is not actually irradiated, it is difficult to evaluate the quality of brightness, chromaticity, illumination uniformity, etc. under the conditions of the rated operation of a completed bulb. The use of this method is thus limited to the discovery of limited types of defects such as the lack of phosphor material or a clogged shadow-mask. Also, with regard to the electron gun, there is no other effective means than the appearance inspection to perceive the lack of phosphor material before completion of the bulb, thus making it difficult to detect defects of the electron emission characteristic at an earlier time.